High-frequency oscillator of the secondary electron-emission type



Mayzoy 1947- c. w. HANsx-:LL 2,420,744

HIGH-FREQUENCY OSCILLATOR OF THE SECONDARY ELECTRONFEMISSION TYPE Filed May 4, 1944 y2 Sheets-Sheet l l E COQ/ /Nc A40/0 /A/ET May 20, 1947. c. w. HANSELL. 2,420,744

HIGH-FREQUENCY OSCILLATOR lOF THE SECONDARY ELECTRON-EMISSION TYPE Filed May 4, 1944 2 sheets-snm 2` l 7'0 Pan/E afar 'e S INVENTOR.

Patented May 20, 1947 HIGH-FREQUENCY OSCILLATOR OF THE SECONDARY ELECTRON -EMISSION TYPE Clarence W. Hansell, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application May 4, 1944, Serial No. 534,066

The present invention relates to improvements in electron multiplier tubes and circuits therefor, and particularly to such tubes having a pair of secondary emissive cathodes between which electrons ow back and forth to produce electron multiplication.

An object of the invention is to provide an improved mechanically rugged electron tube structure .whose individual elements have large surface areas as a result of which the effects of temperature are minimized.

Another object is to provide a vacuum tube structure having a pair of secondary emissive cathodes of large surface area spaced close to each other compared to the diameters of the cathodes, and having an anode of larger diameter than the cathodes surrounding the space between the cathodes.

Another object is to provide a highly efficient high power tube useful for induction heating purposes and capable of generating alternating current power ranging up to hundreds of kilowatts at frequencies from near zero to low radio frequencies up to the order of a megacycle, and particularly useful with high efficiency in the range extending from audio frequencies up to 100,000 cycles.

Another object is to provide a compact, rugged and relatively inexpensive high power tube for use in converting power at one frequency including direct current to power at another frequency.

Another object is to provide a system for obtaining output power at a frequency lower than the excitation power and with reduced losses and smaller dimensioned tubes than would be required if the output power were of the same or a multiple of the input frequency.

Another object is to provide an electron emission switch in which the emission is turned on and oif by turning on and off a radio frequency excitation current.

A further object is to provide an electron emission switch having a pair of secondary emissive cathodes in which the emission is turned off and on by applying excitation continuously in the for-m of radio frequency current and by applying and removing a bias between the cathodes.

A still further object is to provide a vacuum tube having secondary emissive cathode surfaces which are cooler than any other surface exposed to the vacuum, as a result of which deactivation of the secondary emissive cathode surface is prevented.

In the preferred construction of the tube of the invention, the tube is of the evacuated type and 21 Claims. (Cl. Z50-36) employs a pair of secondary emissive cathodes whose diameters are large compared to the spacing therebetween. The cathodes may be circular discs or plates, rectangular or of any suitable configuration, and their active surfaces are cooled so that they are the coolest surfaces exposed to the vacuum. An anode surrounds the cathodes and is made thicker in the region between cathode surfaces than at other places as an aid to conducting away the heat produced on the active anode surfaces.

The close spacing between cathodes as compared .with their diameter makes the production of electron emission substantially independent of the instantaneous anode-to-cathode voltage. If the spacing between cathodes is large, the anodeto-eathode voltage will aifect the electron paths and the electron transit time between cathodes, in which case it would be necessary to start A. C. operation with a low anode-to-cathode voltage and then increase the voltage. By using a small ratio of spacing between cathodes to cathode diameter, in accordance with the preferred construc-tion of the tube of the invention, it is possible to start emission vwhile the anode to cathode potential is high, without dropping temporarily. By making the secondary emissive cathodes the coolest surfaces exposed tothe vacuum, deactivation is prevented, and continued reactivation is continuously assured. Such deactivation might otherwise occur due to the loss from the cathodes of activating material which tends to migrate to cooler surfaces by the process of sublimation. The thicker portions of the anode at the places closest to the cathodes insure better heat dissipation from the anode surface receiving the electrons, and also minimize the number of electrons which might tend to strike the glass seal. By curving certain portions of the anode or by placing target portions between the anode and glass envelope, it is further possible to prevent any electrons from striking the glass. Of course, the anode itself might form part or nearly all of the evacuated envelope of the tube. I

In practice I have found that it is possible by suitable A. C. excitation to cause a growth of electron emission from suitably activated secondary emission cathodes which are completely cold. Apparently there are enough electrons emitted from surfaces due to light, cosmic rays, evaporation and condensation processes or other causes so that, except for a little initial hesitation, growth of emission can be accomplished. Once growth has taken place it appears that a very small emission persists for some time after 3 secondary emission excitation is removed so that starting after bref periods of removal of excitation, is prompt and reliable.

A more detailed description of the invention follows in conjunction with drawings, wherein;

Fig. l illustrates the invention in a high power converter for converting direct or low frequency alternating current to higher frequency current;

Fig. la illustrates the same system as Fig. l, with tuned circuits in the form of sections or transmission line;

Fig. 2 is a modification of Fig. l and illustrates another way of keying the tube than by modulating the radio frequency current; and

Fig. 3 illustrates the tube of the invention used as a switch or circuit breaker'.

Referring to Fig. i in more detail, there is shown my improved evacuated electron discharge device i8 employed as a high power tube in a circuit for converting direct cu rent or low alternating f 'equency current supplied from a power source ll to a higher frequency current utilized for heating induction heating apparatus I2.

rEhe vacuum tube l comprises a pair of secondary emissive cathodes i in the form of plates or discs 'wn-ose confronting surfaces are constituted of, or coated with suitable material to give the best secondary emission possible. Cathodes E are preferably spaced apart only a short distance compared to the diameters of the cathodes, as shown. rThis small spacing is not absolutely essential to obtaining a workable converter but it provides great convenience in operation by making it possible to start and stop operation of the converter without momentarily changing the power supply potential. If the cathodes were designed with a large ratio of spacing to diameter the electric field between anode and cathodes could inhibit the growth of secondary emission.

Surrounding the cathodes is a cylindrical metallic anode 2 having a relatively thick portion in the region adjacent the space between the cathodes. This thick portion, shown as a bump, insures better heat dissipation from the bombarded anode surface and also minimizes the tendency for electrons to flow toward the anode portion adjacent the seal to the glass envelope 3. rThe anode is slightly curved at its extremities fi to prevent electrons from striking the glass. lt will thus be seen that the anode itself forms a large part of the evacuated envelope.

The cathodes are maintained the coolest surfaces of the tube elements in order to prevent deactivation; that is, loss of activating material which might otherwise flow to cooler surfaces by the process of sublimation. This is achieved by means of a flow of cooling fluid, such as water or air, passing in through inlets and out through outlets 8. The inlets and Outlets are in the form of metallic coaxial conduits, as shown.

In practice the activating material may be one or a combination of elements of the most easily ionizable kind condensed or bound on an oxidized or otherwise chemically complex surfee.

There are a number of types of composite surfaces for producing a large ratio of secondary emission electron current due to an impinging electron current. Those having the highest ratio appear to have the common limitation that they are spoiled, at least temporarily, by overheating. I believe that this loss of activation is primarily, though not entirely due to evaporation of activating material from the surface. By reversing the process to cause condensation on the surface, the cooled cathodes can be protected from deactivating and may be continuously activatec;L instead. By this means, goo,r4

operate at a higher temperature surfaces exposed to the vacuum. By hee'L active areas of the cathode at a lower temperature than any other areas exposed to the vac-- uum even volatile caesium can 'se kept on the cathode surfaces.

The cathodes are coupled to opp-e nais of a parallel tuned circuit l wlii o frequency ex itation pote t ones. Tuned circuit l' is coupled cuit forming the output of radio frequency power 9'. ample, be an plying iCG megacycle energy to Source 9 is modulated or modulator 3, which for pulses 'he output of source at a 50,095) cycles per second.

The output of tube [El is coupled to a which is tuned to the 50,300 cycle mod' frequency and which supplies heating induction he in@ apparatus l2.

any desired type of equipment.

' l supplies hign voltage,

A power source l say by way of example only, l5 current or low frequency a sixty cycles by way of illus minals of a condenser connected to the tuned shown.

In the operation of the system of Fig. l, tuned circuit 3 supplies pulses of high frequency energy to tuned circuit l. These pulses are repeated at a E0 lsilocycle rate while the high frequency energy is at l0() megacycles. Both circuits 'l and are tuned to the 1GO inegacycle frequency and, in practice, would consist of sections of transmission line, such as concentric lines rather than the equivalent lumped circuits. This is shown in Fig. icl. This energy excites the secondary sive cathodes l setting up a i megacycle A. C. electric iield between them. The spacing between tho cathode surface is correlated to the excitation radio frequency and the voltage between the cathodes. Obviously, this voltage affects the transit time of the electrons traveling between the cathcdes. This transit time should be of the order or" one-half of the excitation frequency cycle. Electrons will leave either cati` ode and travel to the other cathode under the influence of the applied radio fr quency ield, and only those electrons near the outside edges of the cathodes escape to the anode il. An electron leaving the center of the surface of one cathode will be drawn by the radio frequency iield to the other cathode where it will cause the emission or" several secondary electrons. These secondary electrons will, in turn, cross the space between the cathodes and each produce several additional secondary electrons. This process will be repeated until there are an enormous number of electrons flowing back and forth between the cathodes. At the beginning of this electron building up process, an electron released at the center of one cathode surface will produce an increase in electrons only at or near the center, but as soon as the electrons become numerous a space charge will be built up between the cathodes which will force the phenomena to spread out radially from the center until the whole adjacent surfaces of the cathodes are active in producing emission. When the electrons reach the edges of the cathodes, there will be a continual loss of electrons to the anode and a current will flow from the anode to the cathodes. This current, depending upon the diameter of the cathodes, may easily be as high as 30 amperes and higher. The current carrying capacity of the tube is determined primarily by the circumference of the cathodes. The shape of the cathodes and the anode may be scalloped or made to be irregular to increase this circumference where the greatest possible ratio of anode current to tube dimensions is desired. In general I prefer simpler configurations because they are easier and cheaper to provide.

It should be understood that, for proper emission, the instantaneous anode potential at times when anode current flow is desired, should not be too high and must be properly correlated to the spacing between the cathodes and the cathode diameter so that electrons are not drawn directly to the anode and thus the desired building up of the electrons in the space between the cathodes prevented. In practice, however, such a feature may be relied upon for making and breaking (i. e., interrupting) the emission at the frequency of the output circuit. For example, in a carefully designed tube, the pulling out of the electrons from between the cathodes due to the anode-to-cathode potential, may be so great as to inhibit growth of a substantial electron emission except in time periods when the oscillations in the output circuit have reduced the anode-to-cathode potential to a low value. This will then provide the necessary conditions for self oscillation at the output frequency. With such a self oscillating tube, source 9 may generate continuously and source I3 may be eliminated. For this self oscillating combination, it should be noted, oscillations can start only while the potential of Ithe anode-to-cathode power source is low, but after they have started, the potential may be increased, not too rapidly, to the operating value.

Circuit I4 is tuned to the modulating or keying frequency, in this case 50,000 cycles per second. Output from circuit I4 is at peak voltage about equal to, or somewhat less than the 15,000 volts of power source Il. If power at a lower potential is desired a transformer circuit may be interposed between circuit I4 and the load I2.

Fig. 2 is a modification of the circuit of Fig. 1, and illustrates another way to key or modulate the tube le. The same parts in Figs. 1 and 2 are given the same reference numerals, while the equivalent parts have the same reference numerals with a prime designation. In Fig. 1, the megacycle radio frequency current is supplied to tube I in pulses, while in Fig. 2, the 100 megacycle current is supplied continuously to the tube I0 and reliance is placed on superimposing a lower frequency excitation (the output frequency) between the cathodes upon the 100 megacycle excitation to assist in keying the tube. This superimposing is obtained by a feed back circuit composed of coils I6 which are coupled to the output circuit I4' and connected back to the input circuit 1. The direct current bias from source I1 prevents current from flowing between anode and cathodes by preventing the growth of secondary emission. This direct current bias is balanced out to zero or to a low value once per cycle by the feed back excitation at the time it is desired that anode current should flow. This feed back excitation is the coupling between output circuit III and input` circuit 'I'. When there is zero bias on the cathodes, the direct current from power source flows through tube I0 between anode and cathodes. The direct current flowing in pulses develops the output voltage which is available in the load circuit I8. This load circuit may be any desired type of circuit, such, for example, as induction heating apparatus.

For starting the arrangement of Fig. 2, the bias from source yI'I may be reduced momentarily or, preferably, the bias from I'| is obtained by rectifying a little of the power from circuit I4 to provide a direct current potential which adjusts itself automatically to cause starting and correct conditions during operation. If bias source II is driven from the main power supply it is preferably connected with the same source as l I so that their potentials vary together.

Fig. 3 illustrates the use of the improved tube of the invention as a switch or circuit breaker, capable of handling several hundreds or even one thousand amperes at extremely high D. C. potentials such as might be employed in the long distance transmission of electrical power. Much effort has been put into the development of D. C. transmission because of the economies in line construction which it offers, once the needed auxiliary equipment is available. A source of high voltage anywhere in the range between 20,000 volts and 300,000 volts is represented by box 20. This source may be a power house having conventional alternating current generators and rectiers, and may be located at a remote point relative to the loads. A transmission line TL connects this power house to the converter apparatus 2| through the tube I0. The secondary emissive cathodes of tube I0 are excited from tuned input circuit "I, in turn coupled to high frequency source 9. A pair of leads 22 couple the oscillator source 9 to a power supply which provides the anode voltage for the oscillator 9 or the alternating current power to the rectifier in the oscillator apparatus 9. Power switches 23a and 23h are provide/d in series with the leads 22. An overload relay 25 controls switch 23a. Switch 23D is manually operated. The direct current-to-alternating current converter 2| may be any suitable apparatus such as the Ignitron D. C. to A. C. power connector, or it may be the same type of converter shown in Figs. 1 and 2 utilizing my improved double secondary emissive cathode tube. The output of converter 2| is at low alternating current freq-uency, 60 cycles for example, and supplies various loads. A branch line TL couples TL to other systems similar to that shown to the right of TL.

In the operation of the system of Fig, 3, as soon as power switch 23h is closed, direct current from the power source 20 will flow over line TL and through tube I0 to the converter 2|. In the case of an overload being drawn through converter 2|, the voltage across resistance R will rise so that the winding of relay 25 (which is in parallel with resistance R) will draw enough current to open switch 23a and thus protect the equipment.

The trouble with conventional types of switches of the contact type is that they involve tremendous technical problems when designed to interrupt voltages as high as 200,000 volts. Such conventional switches are, in addition, extremely expensive and have extremely large dimensions. Further, the use of conventional switches requires the replacement of contacts after usage for a little while. The circuit breaker of the present invention of Fig. 3, however is compact, relatively indestructible and inexpensive.

The following are additional advantages provided by the improved tube of the invention: It is of simplified construction compared to the hot cathode tubes of similar rating; and the tube of the invention has much smaller dissipation of power in the form of heat at the cathodes. This last advantage is due to the fact that the improved tube produces emission only as it is needed, whereas the hot cathodes produce emission continuously, and continuously require power for this purpose.

The electron discharge device of the invention, when built with dimensions which are commonly used for transmitting vacuum tubes, can be used to key the output of a rectifier and the input to a radio frequency amplifier at 20,000 volts and a power of 600 kilowatts. Simply turning on and off the 300 to 500 watts of radio frequency cathode input can switch on and off the 600 kw. direct current power in the manner shown generally in Fig. 3 or alternatively, the same thing may be accomplished -by applying or removing a direct current bias between the cathodes. Thus, a tube of the invention may act as a series relay, and when designed in suitable sizes can convert direct current power of thousands of kilowatts into alternating current power at relatively high efficiency. For example, a unit about the size of a railway mercury arc rectifier tank might very well turn on and off several hundred and even 1000 amperes at 300,000 volts with a radio frequency control energy of only about to 50 kilowatts depending upon the secondary emission ratio of the cathode surfaces and the degree of vacuum maintained in the tube, largely due to low cathode temperature condensing volatile materials.

It should be understood that the values mentioned in this specification for the voltages and frequencies are only illustrative and not limitative, and that different values may be used and properly correlated.

I claim:

l. An electron discharge device suitable for producing a high Power output, comprising a pair of spaced cathodes having secondary emissive surfaces facing each other, an anode surrounding the space between said cathodes, a source of high frequency current coupled between said cathodes for exciting them, means for interrupting the emission from said cathode surfaces at a frequency lower than the excitation frequency, and an output circuit coupled to said anode for deriving power at said lower frequency.

2. An electron discharge device suitable for producing an alternating current power output, comprising a pair of parallel spaced secondary emissive cathode surfaces facing each other, an anode surrounding the space between said cathodes, a source of radio frequency current coupled between said cathodes for exciting them, a modulator coupled to said source for modulating the source at a frequency substantially less than said radio excitation frequency, and an output circuit Cil tuned to the modulation frequency coupled to said anode.

3. An electron discharge device suitable for producing a high power output, comprising a pair of parallel spaced secondary emissive cathode surfaces facing each other, an anode surrounding the space between said cathodes, a source of radio frequency current coupled between said cathodes for exciting them, a modulator coupled to said source for modulating the source at a frequency in the range of audio frequencies up to 1,000,000 cycles, and an output circuit tuned to the modulation frequency coupled to said anode.

4. A power system suitable for induction heating, comprising an evacuated electron discharge device having a pair of parallel spaced secondary emissive cathode surfaces facing eah other, an anode surrounding the space between said cathodes, a source of radio frequency current coupled between said cathodes through an input circuit tuned to the frequency of said source, means for keying said source at a frequency substantially less than said radio frequency, as a result of which pulses of radio frequency energy are supplied to said cathodes, said pulses being repeated at the keying frequency, an output circuit tuned to said keying frequency and coupled to said anode, and a power source of direct current or low alternating current having opposite terminals coupled between said input and output tuned circuits.

5. An evacuated electron discharge device suitable for producing high power output, comprising a pair of parallel spaced cathodes having secondary emissive surfaces facing each other, an anode surrounding the space between said cathodes, a tuned input circuit in the form of a section of transmission line coupled between said cathodes, a source of high frequency current coupled to said section of line, said line being tuned to the frequency of said source, means for modulating the output of said tube at a frequency substantially 'different from the frequency of said source, and an output circuit tuned to said modulating frequency coupled to said anode for deriving output from said device.

6. An evacuated electron discharge device suitable for producing high power output, comprising a pair of parallel spaced cathodes having secondary emissive surfaces facing each other, an anode surrounding the space between said cathodes, a tuned input circuit coupled between said cathodes, a source of radio frequency current coupled to said tuned input circuit, said input circuit being tune'd to the frequency of said source, means for interrupting the emission from said cathode surfaces at a frequency substantially lower than said radio frequency, and an output circuit coupled to said anode and tuned to said lower frequency, said interrupting means including a bias source for said cathodes and a feed back circuit between said input and output tuned circuits.

'7. An electron discharge device devoid of a directly heated cathode, comprising a pair of electron impermeable parallel spaced cathode plates having secondary emissive surfaces facing each other, the 4distance between said cathodes being short compared to the diameters of said secondary emissive surfaces, and a single electron collecting anode surrounding the entire space between said cathodes.

8. An electron discharge device comprising a. pair of parallel spaced cathode plates having secondary emissive surfaces facing each other, the

9. distance between said cathodes being short compared to the diameters of said secondary ernissive surfaces, an anode surrounding the space between said cathodes, a source of high frequency current coupled between said cathodes for exciting them, means for interrupting vthe emission from said cathode surfaces at a frequency lower than the excitation frequency, and an output circuit coupled to said anode for deriving power `at said lower frequency, said output circuit being tuned to said lower frequency.

9. An electron discharge device comprising a pair of spaced cathodes having secondary emissive surfaces facing each other and located in an evacuated envelope, an anode surrounding the space between said cathodes, means supplying cooling fluid to said cathodes at such temperatures as to make said secondary emissive surfaces the coolest parts of said device exposed to said vacuum, a source of high frequency current coupled between said cathodes for exciting the same, means for interrupting the emission from said cathode surfaces at a frequency lower than the excitation frequency, and an output circuit coupled to said anode for deriving power at said lower frequency.

l0. An electron discharge device including a disc-like cathode having a surface coated with secondary emissive activating material, an anode surrounding said cathode and enclosing an evacuated space, and means for preventing loss vof activating material from said cathode to other parts of said device comprising cooling fluid inlet and outlet tubes maintaining said surface the coolestl surface exposed .to the vacuum, whereby deactivation of said secondary emissive cathode is prevented, said inlet and outlet tubes having their longitudinal axes substantially perpendicular to the plane of said cathode.

11. An electron discharge device including a pair of parallel spaced cathode plates having their confronting surfaces coated with secondary emissive activating material, said secondary emissive cathodes being located within an evacuated container a pair of inlet tubes entering said container at right anglesto said plates and supplying cooling fluid to said cathodes, and a pair of outlet tubes entering said container at right angles to said plates for removing the cooling fluid after it has cooled said cathodes, whereby said cathodes are maintained the coolest surfaces exposed to the vacuum.

12. An electron discharge device including a pair of parallel spaced cathode plates having their confronting surfaces coated with secondary emissive activating material, said secondary emissive cathodes being located within an evacuated container, an inlet tube and a coaxial outlet tube for supplying cooling fluid to each cathode, said tubes entering said container and being sealed thereto, the coaxial tubes for said cathodes entering said container frorn opposite sides thereof.

13. A high power converter system of direct current or low alternating current to energy in the frequency range from audio to 1,000,000 cycles, comprising a vacuum tube having a pair of parallel spaced cathode plates whose confronting surfaces are coated with secondary einissive activating material, the spacing between said plates being shorter than the diameters of said plates, an anode surrounding the space between said cathode plates, means for maintaining said cathodes the coolest surfaces exposed to the vacuum, a tuned input circuit coupled between said cathodes, a source of radio frequency excitation current coupled to said tuned input circuit, said input circuit being tuned to said radio frequency, means for modulating said tube at a frequency substantially lower than said radio frequency, an output circuit coupled to said anode and tuned to said modulating frequency, a power source of direct current or low frequency alternating current having opposite terminals connected from a direct current standpoint to said input and output tuned circuits, whereby said vacuum tube produces power output at a peak potential close to the potential of said power source and at said modulating frequency.

14. An electronv discharge device devoid of a directly heated cathode, comprising a pair of electron impermeable parallel spaced cathode plates having coated secondary ernissive surfaces facing each other, the distance between said cathodes being short compared to the diameters of said secondary emissive surfaces, an anode surrounding the entire space between said cathodes, said space being evacuated, that portion of the anode adjacent said space being thicker than the other portions of said anode.

15. An electron discharge device comprising a pair of parallel spaced cathode plates having coated secondary emissive surfaces facing each other, the distance between said cathodes being short compared to the diameters of said second.. ary emissive surfaces, an anode surrounding the space between said cathodes, said space being evacuated, a tuned circuit coupled between said cathodes, and a source of radio frequency current coupled to said tuned circuit, said tuned circuit being tuned to said radio frequency.

16. An electronic switch between a power source of high voltage and a load, comprising an electron discharge device having a pair of parallel spaced secondary ernissive cathode surfaces facing each other, the distance between said cathodes being short compared to the diameter of said secondary emissive surfaces, an anode surrounding the space between said cathodes, said space being evacuated, a transmission line coupling said source of high voltageto said anode, a source of radio frequency control energy, a circuit tuned to said radio frequency coupled between said cathodes and also coupled to said radio frequency source, and a load coupled to said cathodes.

17. An electronic switch between a power source of high voltage and a load, comprising an electron discharge device having a pair of parallel spaced secondary eniissive cathode surfaces facing each other, the distance between said cathodes being short compared to the diameter of said secondary einissive surfaces, an anode surrounding the space between said cathodes, said space being evacuated, a transmission line coupling said source of high voltage to said anode, a source of radio frequency control energy, a circuit tuned to said radio frequency coupled between said cathodes and also coupled to said radio frequency source, a converter of direct current-to-low frequency alternating current coupled between said cathodes and a load.

18. An oscillator comprising an evacuated electron discharge device having impermeable platelike cathodes with secondary ernissive coated sur.. faces facing each other, a source of very high frequency potential coupled between said cathodes and adjusted to obtain a growth of secondary emission from the cathodes, the distance between said cathodes being short compared to the diameter of said secondary emissive surfaces, an electron collecting anode surrounding the entire space between the cathodes to provide an electric eld which can inhibit substantial emission when the instantaneous anode-to-cathode potential is high but which permits growth of emission when the anode-to-cathode potential is low, thereby providing the conditions for self oscillation.

19. An electron discharge device suitable for producing a high power outputy comprising a pair of spaced plate-like cathodes having secondary emissive surfaces facing each other, the distance between said cathodes being short compared to the distance across the secondary emissive surface area of each of said secondary emissive surfaces, an anode surrounding the space between said cathodes, a source of high frequency current coupled between said cathodes for exciting them, means for interrupting the emission from said cathode surfaces at a frequency lower than the excitation frequency, and an output circuit coupled to said anode for deriving power at said lower frequency.

20. An electron discharge device including a pair of parallel spaced cathode plates having their confronting surfaces coated with secondary emissive activating material, the distance between said plates being short compared to the distance across the secondary emissive surface area of each of said plates, said secondary emissive cathodes being located within an evacuated container', at least one inlet tube entering said container 12 and supplying cooling uid to said cathodes, and at least one outlet tube entering said container for removing the cooling fluid after it has cooled said cathodes, whereby said cathodes are maintained the coolest surfaces exposed to the vacuum.

21. An electric tube having a cathode within and spaced from a surrounding envelope, said cathode having a. coating of secondary emissive material thereon, and means for preventing loss of activating material from said coating comprising means for cooling said cathode to maintain it at a lower temperature than any other portion of the tube within said envelope.

CLARENCE W. HANSELL,

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,026,892 Heintz Jan. 7, 1936 2,298,673 Brown Oct. 13, 1942 2,138,896 Yonkers Dec. 6, 1938 2,141,837 Farnsworth Dec. 27, 1938 2,171,214 Jonker Aug. 29, 1939 2,216,169 George Oct. 1, 1940 2,037,977 Hansell Apr. 21, 1936 

